Hello Mike. You are correct - checked at the stall is the true reading! I was flying a rented Russian glider in the women's European Championships a few years ago. We had a lovely evening before, celebrating with other participants....and I was not as careful as should have been reviewing my ground crew's preparations! On being towed up by a Wilga on pre competition practice, the ASI read initially zero, then wound round the dial to the stop!
The airtow was progressing normally, so I released at usual height (still slightly confused by all information being in meters) and did a series of gentle stalls. Instruments still no help, but the attitude was sufficient to recognise safe speed. All the same my ground crew came rushing up after I landed to say "too fast! too fast!"
Better than too slow, I reckoned.